In some applications, a portion or all of the liquid nitrogen product from a cryogenic air separation plant is required at a purity higher than normal. More specifically, the shelf liquid, which is the source of the liquid nitrogen product, is required to contain less than 100 parts per million (ppm) argon and possibly as little as 10 ppm argon. The typical argon concentration in the shelf liquid nitrogen of a conventional cryogenic air separation plant is more than 1000 ppm when the oxygen content of the shelf is maintained at about 1 ppm. Heretofore, in order to reduce the argon content of the shelf, the reflux ratio (L/V) within the higher pressure column must be increased.
The conventional way of increasing L/V without altering the total higher pressure column stage count is to increase the purity specification of shelf nitrogen, i.e. reduce its oxygen content below 1 ppm, to the value that conforms with the required argon specification. This is accomplished by significantly decreasing the flow rate of shelf liquid that is used as reflux in the lower pressure column. As a result, the overall argon recovery is decreased. Attempts to compensate for the loss of argon recovery due to lack of reflux to the lower pressure column by adding liquid to the top of the lower pressure column to make up for the reduced liquid reflux cannot improve argon recovery beyond conventional levels.
Accordingly it is an object of this invention to provide a cryogenic rectification system which produces nitrogen having a low argon content without losing a significant amount of reflux for the lower pressure column.
It is another object of this invention to provide a cryogenic rectification system which enables an overall improvement is argon recovery.